Riser reactors provide a convenient reaction system for performing gas-solids reactions. Gas phase reactants can be brought into contact with solid catalyst while inside the riser to convert the reactants into a desired product. Upon exiting the riser, the conversion product is separated from the solids to recover the catalyst.
In reaction systems where only a fraction of the solid catalyst is regenerated during each loop through the system, product vapors generated during one pass through the standpipe or riser reactor can stay with the catalyst during the next pass through the riser reactor. Passing the product vapors through the riser reactor a second time can lead to excess reaction, which can lead to undesirable products and possibly reduce the operating efficiency of the reaction system.
The above problems are further compounded in multiple riser systems. In order to maintain control over the gas-solids reaction, the average state of the solid catalyst in each riser must be the same. To achieve this, a roughly equal amount of regenerated and non-regenerated catalyst must be introduced into each of the multiple risers.
U.S. Pat. No. 4,364,905 describes a single riser FCC reaction system for improving the separation of entrained product vapors from spent catalyst. A stripping gas is flowed through the spent catalyst as it passes through a baffled region. All of the catalyst is then sent to a regenerator.
What is needed is a system and method for improving the separation of gas phase conversion product from catalyst solids. The system and method should be compatible with reaction systems having multiple risers. The system and method should also allow for roughly even distribution of regenerated and non-regenerated catalyst particles into the multiple risers.